The invention relates to a method for selecting or changing communications modes used by a terminal on a communications connection over a packet-switched data network where the communications mode can be based on either real-time or non-real-time transfer and where the service capacity varies locally or temporally. The invention further relates to a communications network arrangement utilizing the method, a communications network server, a cellular terminal, and a software means stored at a terminal.
Circuit-switched bi-directional so-called duplex voice connections over telephone networks have been commonplace for over 100 years. People communicating over a voice connection can speak and listen to one another simultaneously because the communications link is active in both directions continuously. As one party stops speaking, the other party can continue at any desired point of time because the circuit-switched connection is not cut off during the call. The connection is terminated only when the parties so decide. As far as data communication is concerned a circuit-switched connection is relatively inefficient as it reserves a communication link also at those time when there is no data to be communicated.
Data connections, just as voice connections, are increasingly turning from circuit-switched connections into packet-switched connections. One advantage of packet-switched networks is the efficient use of communications resources. The best known packet-switched network among users is the Internet. Another known packet-switched service/network is the GPRS (General Packet Radio Service) which was originally created to serve data communication needs. In a GPRS network, a communications connection between two terminals is established only when there exists a need to transfer data. Such a connection thus resembles a so-called simplex connection known from the prior art. An individual communications connection operates only in one direction at a time, either downlink towards the terminal, or uplink towards the backbone network. In GPRS, however, a terminal may have simultaneous uplink and downlink connections. When the need to communicate ceases completely or when there are momentarily no more data packets to transmit, the dedicated channel used is removed. If necessary, this release of the link can be delayed by a few hundred milliseconds typically.
Voice communication over a packet-switched GPRS network requires the utilization of several different communications protocols. Such required communications protocols include, for instance, the RTP (Real Time Protocol), UDP (User Datagram Protocol), and IP (Internet Protocol). Of these, the RTP is the one that sees to that voice packets are delivered in the correct order and with as short delay as possible to the receiving terminal.
A conversation between two people usually proceeds in ‘bursts’. Therefore, breaks in speech spurts or a changeover from one speaker to another may result in a break in the data transfer. A changeover between speakers additionally always involves a reaction time from the part of the person who was listening, before he starts talking himself, which may result in a situation in which the dedicated channel reserved for the preceding speech burst is already released a few hundred milliseconds after the preceding speech burst. Termination of a radio link in a conventional GPRS network is controlled using a so-called TBF procedure (Temporary Bit Flow). In TBF, a dedicated channel can be maintained for a while after a break in the data transferred. So, when data run out in the memory of the transmitting terminal, the TBF procedure typically releases the dedicated channel in a few hundred milliseconds.
If, however, a new dedicated channel for new data to be transferred is required in this situation, it must be established through a slower process by first utilizing a common control channel of the GPRS network. This takes several hundred milliseconds. At worst this chain of events, disconnection and re-connection, may cause pauses of 0.5 to 1.5 seconds over the connection, which pauses are perceived by the user. These pauses substantially affect the user's communication.
In the TBF procedure of the GPRS service it is possible to increase the TBF release delay in both directions, uplink and downlink. This can be done in an attempt to improve the functionality of the GPRS network for voice connections. In the downlink direction a so-called delayed TBF procedure is used, and in the uplink direction, a so-called extended TBF procedure. A typical TBF release delay is of the order of 1000 to 2000 ms. With these procedures, the link between the terminal and base station can be maintained for a while without data being transferred. A longer TBF release would result in the battery of the terminal to discharge quickly.
The basic service capabilities of GPRS networks vary a lot. In some GPRS networks with a good performance the users can be guaranteed almost real-time streaming mode voice connections. In such connections, speech is buffered at the receiver into blocks of 0.5 seconds at most before being played back to the user. In this kind of GPRS networks the service level of the connection must be well defined and controlled. In such networks the terminals and the network agree on the Quality of Service (QoS) level before the actual establishment of the connection. Communication parameters to be agreed upon may include e.g. the maximum allowable latency time between terminals, the communications capacity required on the link, or the quantity of errors allowed. In the provision of the QoS required for the streaming mode in GPRS networks, problems are especially caused by radio path fade-outs or momentary network overloads which may cut off the communication link or force serving cell handovers in rapid succession.
Currently, however, there are many GPRS networks that for some reason or another are not capable of the functionality required by the QoS service described above. The reason may be that some necessary function is missing or that the communications capacity of the network is insufficient in general. In such GPRS networks, a certain amount of speech, say 5 seconds, is first stored in a buffer memory at the receiver's terminal from where it is then played back to the receiver. This is a non-real-time store-and-play mode. Such a voice connection cannot be a duplex voice connection but, instead, each user must be ready to adapt to even relatively long delays before starting their own speech spurt or hearing the answer. From the user's point of view this is an undesirable feature which should be avoided when possible. The store-and-play connection or service is not susceptible to problems associated with the streaming mode. Therefore it can be always used in any GPRS network in any kind of communications conditions.
One possible connection set-up technique in a packet-switched GPRS network is a so-called PoC session (Push to talk over Cellular). Several people may participate simultaneously in a PoC session and they can communicate through speech or SMS messages. The participants take turns speaking, and the direction of transmission varies. A PoC session can utilize either one of the connection techniques described above. Naturally, from the point of view of the user, the streaming mode connection/service is the more desirable one. The PoC service will hereinafter be called VSS (Voice Streaming Service) in this patent application in order to reflect the fact that the primary operating mode of a voice connection is to utilize the streaming mode service.
When it is desired to establish a real-time voice connection, such as a VSS, there cannot be certainty about whether the serving GPRS network in its completeness is able to support the QoS required. The problem is particularly emphasized at the B subscriber's, or the receiving, terminal which has to adapt to that which is provided to it by the serving network. Even if the sending terminal, A subscriber, were able to transmit using streaming mode, a poor service quality in the rest of the network may force the receiver, B subscriber, to receive according to the store-and-play mode. A connection may be attempted based on the assumption that the GPRS network as a whole supports real-time VSS. If this is not the case, the user of the terminal must suddenly adapt to an alternative and undesirable situation. On the other hand, if the network in question supports streaming mode in VSS, and the user does not know that, he may default to use a service according to the store-and-play method. It can be assumed, however, that users want to use streaming mode with voice connections. Unfortunately, GPRS networks according to the prior art are still relatively bad performers as regards functionality for voice connections.